This product is simply amazing. It took me about 5 minutes to install.
It eliminates that moving bolt on the side of the gun, which while it looks really cool, hurts like hell if you get your thumb caught. It also eliminates snagging on the bolt.

It protects the markers internals from paint and dirt getting in, so thats an obvious plus.

Conclusion:

Because it costs so little and does so much, I'll give it a 10. I can't think of anything that would make it better.